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author | zzak <zzak@b2dd03c8-39d4-4d8f-98ff-823fe69b080e> | 2013-06-11 03:50:33 +0000 |
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committer | zzak <zzak@b2dd03c8-39d4-4d8f-98ff-823fe69b080e> | 2013-06-11 03:50:33 +0000 |
commit | df4abd9b3b4d5ac061f098adedbf185ecd223909 (patch) | |
tree | 5c5efcc6d4583cb5b261f67b61c0e453fd472a80 /doc/regexp.rdoc | |
parent | c0ce87736511ca51e453509c25fe3c00abb08059 (diff) | |
download | ruby-df4abd9b3b4d5ac061f098adedbf185ecd223909.tar.gz |
* doc/re.rdoc: Rename to doc/regexp.rdoc
* re.c: Update rdoc include for rename of file
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41225 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Diffstat (limited to 'doc/regexp.rdoc')
-rw-r--r-- | doc/regexp.rdoc | 653 |
1 files changed, 653 insertions, 0 deletions
diff --git a/doc/regexp.rdoc b/doc/regexp.rdoc new file mode 100644 index 0000000000..1c68533a8d --- /dev/null +++ b/doc/regexp.rdoc @@ -0,0 +1,653 @@ +# -*- mode: rdoc; coding: utf-8; fill-column: 74; -*- + +Regular expressions (<i>regexp</i>s) are patterns which describe the +contents of a string. They're used for testing whether a string contains a +given pattern, or extracting the portions that match. They are created +with the <tt>/</tt><i>pat</i><tt>/</tt> and +<tt>%r{</tt><i>pat</i><tt>}</tt> literals or the <tt>Regexp.new</tt> +constructor. + +A regexp is usually delimited with forward slashes (<tt>/</tt>). For +example: + + /hay/ =~ 'haystack' #=> 0 + /y/.match('haystack') #=> #<MatchData "y"> + +If a string contains the pattern it is said to <i>match</i>. A literal +string matches itself. + + # 'haystack' does not contain the pattern 'needle', so doesn't match. + /needle/.match('haystack') #=> nil + # 'haystack' does contain the pattern 'hay', so it matches + /hay/.match('haystack') #=> #<MatchData "hay"> + +Specifically, <tt>/st/</tt> requires that the string contains the letter +_s_ followed by the letter _t_, so it matches _haystack_, also. + +== <tt>=~</tt> and Regexp#match + +Pattern matching may be achieved by using <tt>=~</tt> operator or Regexp#match +method. + +=== <tt>=~</tt> operator + +<tt>=~</tt> is Ruby's basic pattern-matching operator. When one operand is a +regular expression and the other is a string then the regular expression is +used as a pattern to match against the string. (This operator is equivalently +defined by Regexp and String so the order of String and Regexp do not matter. +Other classes may have different implementations of <tt>=~</tt>.) If a match +is found, the operator returns index of first match in string, otherwise it +returns +nil+. + + /hay/ =~ 'haystack' #=> 0 + 'haystack' =~ /hay/ #=> 0 + /a/ =~ 'haystack' #=> 1 + /u/ =~ 'haystack' #=> nil + +Using <tt>=~</tt> operator with a String and Regexp the <tt>$~</tt> global +variable is set after a successful match. <tt>$~</tt> holds a MatchData +object. Regexp.last_match is equivalent to <tt>$~</tt>. + +=== Regexp#match method + +#match method return a MatchData object : + + /st/.match('haystack') #=> #<MatchData "st"> + +== Metacharacters and Escapes + +The following are <i>metacharacters</i> <tt>(</tt>, <tt>)</tt>, +<tt>[</tt>, <tt>]</tt>, <tt>{</tt>, <tt>}</tt>, <tt>.</tt>, <tt>?</tt>, +<tt>+</tt>, <tt>*</tt>. They have a specific meaning when appearing in a +pattern. To match them literally they must be backslash-escaped. To match +a backslash literally backslash-escape that: <tt>\\\\\\</tt>. + + /1 \+ 2 = 3\?/.match('Does 1 + 2 = 3?') #=> #<MatchData "1 + 2 = 3?"> + +Patterns behave like double-quoted strings so can contain the same +backslash escapes. + + /\s\u{6771 4eac 90fd}/.match("Go to 東京都") + #=> #<MatchData " 東京都"> + +Arbitrary Ruby expressions can be embedded into patterns with the +<tt>#{...}</tt> construct. + + place = "東京都" + /#{place}/.match("Go to 東京都") + #=> #<MatchData "東京都"> + +== Character Classes + +A <i>character class</i> is delimited with square brackets (<tt>[</tt>, +<tt>]</tt>) and lists characters that may appear at that point in the +match. <tt>/[ab]/</tt> means _a_ or _b_, as opposed to <tt>/ab/</tt> which +means _a_ followed by _b_. + + /W[aeiou]rd/.match("Word") #=> #<MatchData "Word"> + +Within a character class the hyphen (<tt>-</tt>) is a metacharacter +denoting an inclusive range of characters. <tt>[abcd]</tt> is equivalent +to <tt>[a-d]</tt>. A range can be followed by another range, so +<tt>[abcdwxyz]</tt> is equivalent to <tt>[a-dw-z]</tt>. The order in which +ranges or individual characters appear inside a character class is +irrelevant. + + /[0-9a-f]/.match('9f') #=> #<MatchData "9"> + /[9f]/.match('9f') #=> #<MatchData "9"> + +If the first character of a character class is a caret (<tt>^</tt>) the +class is inverted: it matches any character _except_ those named. + + /[^a-eg-z]/.match('f') #=> #<MatchData "f"> + +A character class may contain another character class. By itself this +isn't useful because <tt>[a-z[0-9]]</tt> describes the same set as +<tt>[a-z0-9]</tt>. However, character classes also support the <tt>&&</tt> +operator which performs set intersection on its arguments. The two can be +combined as follows: + + /[a-w&&[^c-g]z]/ # ([a-w] AND ([^c-g] OR z)) + # This is equivalent to: + /[abh-w]/ + +The following metacharacters also behave like character classes: + +* <tt>/./</tt> - Any character except a newline. +* <tt>/./m</tt> - Any character (the +m+ modifier enables multiline mode) +* <tt>/\w/</tt> - A word character (<tt>[a-zA-Z0-9_]</tt>) +* <tt>/\W/</tt> - A non-word character (<tt>[^a-zA-Z0-9_]</tt>) +* <tt>/\d/</tt> - A digit character (<tt>[0-9]</tt>) +* <tt>/\D/</tt> - A non-digit character (<tt>[^0-9]</tt>) +* <tt>/\h/</tt> - A hexdigit character (<tt>[0-9a-fA-F]</tt>) +* <tt>/\H/</tt> - A non-hexdigit character (<tt>[^0-9a-fA-F]</tt>) +* <tt>/\s/</tt> - A whitespace character: <tt>/[ \t\r\n\f]/</tt> +* <tt>/\S/</tt> - A non-whitespace character: <tt>/[^ \t\r\n\f]/</tt> + +POSIX <i>bracket expressions</i> are also similar to character classes. +They provide a portable alternative to the above, with the added benefit +that they encompass non-ASCII characters. For instance, <tt>/\d/</tt> +matches only the ASCII decimal digits (0-9); whereas <tt>/[[:digit:]]/</tt> +matches any character in the Unicode _Nd_ category. + +* <tt>/[[:alnum:]]/</tt> - Alphabetic and numeric character +* <tt>/[[:alpha:]]/</tt> - Alphabetic character +* <tt>/[[:blank:]]/</tt> - Space or tab +* <tt>/[[:cntrl:]]/</tt> - Control character +* <tt>/[[:digit:]]/</tt> - Digit +* <tt>/[[:graph:]]/</tt> - Non-blank character (excludes spaces, control + characters, and similar) +* <tt>/[[:lower:]]/</tt> - Lowercase alphabetical character +* <tt>/[[:print:]]/</tt> - Like [:graph:], but includes the space character +* <tt>/[[:punct:]]/</tt> - Punctuation character +* <tt>/[[:space:]]/</tt> - Whitespace character (<tt>[:blank:]</tt>, newline, + carriage return, etc.) +* <tt>/[[:upper:]]/</tt> - Uppercase alphabetical +* <tt>/[[:xdigit:]]/</tt> - Digit allowed in a hexadecimal number (i.e., + 0-9a-fA-F) + +Ruby also supports the following non-POSIX character classes: + +* <tt>/[[:word:]]/</tt> - A character in one of the following Unicode + general categories _Letter_, _Mark_, _Number_, + <i>Connector_Punctuation</i> +* <tt>/[[:ascii:]]/</tt> - A character in the ASCII character set + + # U+06F2 is "EXTENDED ARABIC-INDIC DIGIT TWO" + /[[:digit:]]/.match("\u06F2") #=> #<MatchData "\u{06F2}"> + /[[:upper:]][[:lower:]]/.match("Hello") #=> #<MatchData "He"> + /[[:xdigit:]][[:xdigit:]]/.match("A6") #=> #<MatchData "A6"> + +== Repetition + +The constructs described so far match a single character. They can be +followed by a repetition metacharacter to specify how many times they need +to occur. Such metacharacters are called <i>quantifiers</i>. + +* <tt>*</tt> - Zero or more times +* <tt>+</tt> - One or more times +* <tt>?</tt> - Zero or one times (optional) +* <tt>{</tt><i>n</i><tt>}</tt> - Exactly <i>n</i> times +* <tt>{</tt><i>n</i><tt>,}</tt> - <i>n</i> or more times +* <tt>{,</tt><i>m</i><tt>}</tt> - <i>m</i> or less times +* <tt>{</tt><i>n</i><tt>,</tt><i>m</i><tt>}</tt> - At least <i>n</i> and + at most <i>m</i> times + + # At least one uppercase character ('H'), at least one lowercase + # character ('e'), two 'l' characters, then one 'o' + "Hello".match(/[[:upper:]]+[[:lower:]]+l{2}o/) #=> #<MatchData "Hello"> + +Repetition is <i>greedy</i> by default: as many occurrences as possible +are matched while still allowing the overall match to succeed. By +contrast, <i>lazy</i> matching makes the minimal amount of matches +necessary for overall success. A greedy metacharacter can be made lazy by +following it with <tt>?</tt>. + + # Both patterns below match the string. The first uses a greedy + # quantifier so '.+' matches '<a><b>'; the second uses a lazy + # quantifier so '.+?' matches '<a>'. + /<.+>/.match("<a><b>") #=> #<MatchData "<a><b>"> + /<.+?>/.match("<a><b>") #=> #<MatchData "<a>"> + +A quantifier followed by <tt>+</tt> matches <i>possessively</i>: once it +has matched it does not backtrack. They behave like greedy quantifiers, +but having matched they refuse to "give up" their match even if this +jeopardises the overall match. + +== Capturing + +Parentheses can be used for <i>capturing</i>. The text enclosed by the +<i>n</i><sup>th</sup> group of parentheses can be subsequently referred to +with <i>n</i>. Within a pattern use the <i>backreference</i> +<tt>\n</tt>; outside of the pattern use +<tt>MatchData[</tt><i>n</i><tt>]</tt>. + + # 'at' is captured by the first group of parentheses, then referred to + # later with \1 + /[csh](..) [csh]\1 in/.match("The cat sat in the hat") + #=> #<MatchData "cat sat in" 1:"at"> + # Regexp#match returns a MatchData object which makes the captured + # text available with its #[] method. + /[csh](..) [csh]\1 in/.match("The cat sat in the hat")[1] #=> 'at' + +Capture groups can be referred to by name when defined with the +<tt>(?<</tt><i>name</i><tt>>)</tt> or <tt>(?'</tt><i>name</i><tt>')</tt> +constructs. + + /\$(?<dollars>\d+)\.(?<cents>\d+)/.match("$3.67") + => #<MatchData "$3.67" dollars:"3" cents:"67"> + /\$(?<dollars>\d+)\.(?<cents>\d+)/.match("$3.67")[:dollars] #=> "3" + +Named groups can be backreferenced with <tt>\k<</tt><i>name</i><tt>></tt>, +where _name_ is the group name. + + /(?<vowel>[aeiou]).\k<vowel>.\k<vowel>/.match('ototomy') + #=> #<MatchData "ototo" vowel:"o"> + +*Note*: A regexp can't use named backreferences and numbered +backreferences simultaneously. + +When named capture groups are used with a literal regexp on the left-hand +side of an expression and the <tt>=~</tt> operator, the captured text is +also assigned to local variables with corresponding names. + + /\$(?<dollars>\d+)\.(?<cents>\d+)/ =~ "$3.67" #=> 0 + dollars #=> "3" + +== Grouping + +Parentheses also <i>group</i> the terms they enclose, allowing them to be +quantified as one <i>atomic</i> whole. + + # The pattern below matches a vowel followed by 2 word characters: + # 'aen' + /[aeiou]\w{2}/.match("Caenorhabditis elegans") #=> #<MatchData "aen"> + # Whereas the following pattern matches a vowel followed by a word + # character, twice, i.e. <tt>[aeiou]\w[aeiou]\w</tt>: 'enor'. + /([aeiou]\w){2}/.match("Caenorhabditis elegans") + #=> #<MatchData "enor" 1:"or"> + +The <tt>(?:</tt>...<tt>)</tt> construct provides grouping without +capturing. That is, it combines the terms it contains into an atomic whole +without creating a backreference. This benefits performance at the slight +expense of readability. + + # The group of parentheses captures 'n' and the second 'ti'. The + # second group is referred to later with the backreference \2 + /I(n)ves(ti)ga\2ons/.match("Investigations") + #=> #<MatchData "Investigations" 1:"n" 2:"ti"> + # The first group of parentheses is now made non-capturing with '?:', + # so it still matches 'n', but doesn't create the backreference. Thus, + # the backreference \1 now refers to 'ti'. + /I(?:n)ves(ti)ga\1ons/.match("Investigations") + #=> #<MatchData "Investigations" 1:"ti"> + +=== Atomic Grouping + +Grouping can be made <i>atomic</i> with +<tt>(?></tt><i>pat</i><tt>)</tt>. This causes the subexpression <i>pat</i> +to be matched independently of the rest of the expression such that what +it matches becomes fixed for the remainder of the match, unless the entire +subexpression must be abandoned and subsequently revisited. In this +way <i>pat</i> is treated as a non-divisible whole. Atomic grouping is +typically used to optimise patterns so as to prevent the regular +expression engine from backtracking needlessly. + + # The <tt>"</tt> in the pattern below matches the first character of + # the string, then <tt>.*</tt> matches <i>Quote"</i>. This causes the + # overall match to fail, so the text matched by <tt>.*</tt> is + # backtracked by one position, which leaves the final character of the + # string available to match <tt>"</tt> + /".*"/.match('"Quote"') #=> #<MatchData "\"Quote\""> + # If <tt>.*</tt> is grouped atomically, it refuses to backtrack + # <i>Quote"</i>, even though this means that the overall match fails + /"(?>.*)"/.match('"Quote"') #=> nil + +== Subexpression Calls + +The <tt>\g<</tt><i>name</i><tt>></tt> syntax matches the previous +subexpression named _name_, which can be a group name or number, again. +This differs from backreferences in that it re-executes the group rather +than simply trying to re-match the same text. + + # Matches a <i>(</i> character and assigns it to the <tt>paren</tt> + # group, tries to call that the <tt>paren</tt> sub-expression again + # but fails, then matches a literal <i>)</i>. + /\A(?<paren>\(\g<paren>*\))*\z/ =~ '()' + + + /\A(?<paren>\(\g<paren>*\))*\z/ =~ '(())' #=> 0 + # ^1 + # ^2 + # ^3 + # ^4 + # ^5 + # ^6 + # ^7 + # ^8 + # ^9 + # ^10 + +1. Matches at the beginning of the string, i.e. before the first + character. +2. Enters a named capture group called <tt>paren</tt> +3. Matches a literal <i>(</i>, the first character in the string +4. Calls the <tt>paren</tt> group again, i.e. recurses back to the + second step +5. Re-enters the <tt>paren</tt> group +6. Matches a literal <i>(</i>, the second character in the + string +7. Try to call <tt>paren</tt> a third time, but fail because + doing so would prevent an overall successful match +8. Match a literal <i>)</i>, the third character in the string. + Marks the end of the second recursive call +9. Match a literal <i>)</i>, the fourth character in the string +10. Match the end of the string + +== Alternation + +The vertical bar metacharacter (<tt>|</tt>) combines two expressions into +a single one that matches either of the expressions. Each expression is an +<i>alternative</i>. + + /\w(and|or)\w/.match("Feliformia") #=> #<MatchData "form" 1:"or"> + /\w(and|or)\w/.match("furandi") #=> #<MatchData "randi" 1:"and"> + /\w(and|or)\w/.match("dissemblance") #=> nil + +== Character Properties + +The <tt>\p{}</tt> construct matches characters with the named property, +much like POSIX bracket classes. + +* <tt>/\p{Alnum}/</tt> - Alphabetic and numeric character +* <tt>/\p{Alpha}/</tt> - Alphabetic character +* <tt>/\p{Blank}/</tt> - Space or tab +* <tt>/\p{Cntrl}/</tt> - Control character +* <tt>/\p{Digit}/</tt> - Digit +* <tt>/\p{Graph}/</tt> - Non-blank character (excludes spaces, control + characters, and similar) +* <tt>/\p{Lower}/</tt> - Lowercase alphabetical character +* <tt>/\p{Print}/</tt> - Like <tt>\p{Graph}</tt>, but includes the space character +* <tt>/\p{Punct}/</tt> - Punctuation character +* <tt>/\p{Space}/</tt> - Whitespace character (<tt>[:blank:]</tt>, newline, + carriage return, etc.) +* <tt>/\p{Upper}/</tt> - Uppercase alphabetical +* <tt>/\p{XDigit}/</tt> - Digit allowed in a hexadecimal number (i.e., 0-9a-fA-F) +* <tt>/\p{Word}/</tt> - A member of one of the following Unicode general + category <i>Letter</i>, <i>Mark</i>, <i>Number</i>, + <i>Connector\_Punctuation</i> +* <tt>/\p{ASCII}/</tt> - A character in the ASCII character set +* <tt>/\p{Any}/</tt> - Any Unicode character (including unassigned + characters) +* <tt>/\p{Assigned}/</tt> - An assigned character + +A Unicode character's <i>General Category</i> value can also be matched +with <tt>\p{</tt><i>Ab</i><tt>}</tt> where <i>Ab</i> is the category's +abbreviation as described below: + +* <tt>/\p{L}/</tt> - 'Letter' +* <tt>/\p{Ll}/</tt> - 'Letter: Lowercase' +* <tt>/\p{Lm}/</tt> - 'Letter: Mark' +* <tt>/\p{Lo}/</tt> - 'Letter: Other' +* <tt>/\p{Lt}/</tt> - 'Letter: Titlecase' +* <tt>/\p{Lu}/</tt> - 'Letter: Uppercase +* <tt>/\p{Lo}/</tt> - 'Letter: Other' +* <tt>/\p{M}/</tt> - 'Mark' +* <tt>/\p{Mn}/</tt> - 'Mark: Nonspacing' +* <tt>/\p{Mc}/</tt> - 'Mark: Spacing Combining' +* <tt>/\p{Me}/</tt> - 'Mark: Enclosing' +* <tt>/\p{N}/</tt> - 'Number' +* <tt>/\p{Nd}/</tt> - 'Number: Decimal Digit' +* <tt>/\p{Nl}/</tt> - 'Number: Letter' +* <tt>/\p{No}/</tt> - 'Number: Other' +* <tt>/\p{P}/</tt> - 'Punctuation' +* <tt>/\p{Pc}/</tt> - 'Punctuation: Connector' +* <tt>/\p{Pd}/</tt> - 'Punctuation: Dash' +* <tt>/\p{Ps}/</tt> - 'Punctuation: Open' +* <tt>/\p{Pe}/</tt> - 'Punctuation: Close' +* <tt>/\p{Pi}/</tt> - 'Punctuation: Initial Quote' +* <tt>/\p{Pf}/</tt> - 'Punctuation: Final Quote' +* <tt>/\p{Po}/</tt> - 'Punctuation: Other' +* <tt>/\p{S}/</tt> - 'Symbol' +* <tt>/\p{Sm}/</tt> - 'Symbol: Math' +* <tt>/\p{Sc}/</tt> - 'Symbol: Currency' +* <tt>/\p{Sc}/</tt> - 'Symbol: Currency' +* <tt>/\p{Sk}/</tt> - 'Symbol: Modifier' +* <tt>/\p{So}/</tt> - 'Symbol: Other' +* <tt>/\p{Z}/</tt> - 'Separator' +* <tt>/\p{Zs}/</tt> - 'Separator: Space' +* <tt>/\p{Zl}/</tt> - 'Separator: Line' +* <tt>/\p{Zp}/</tt> - 'Separator: Paragraph' +* <tt>/\p{C}/</tt> - 'Other' +* <tt>/\p{Cc}/</tt> - 'Other: Control' +* <tt>/\p{Cf}/</tt> - 'Other: Format' +* <tt>/\p{Cn}/</tt> - 'Other: Not Assigned' +* <tt>/\p{Co}/</tt> - 'Other: Private Use' +* <tt>/\p{Cs}/</tt> - 'Other: Surrogate' + +Lastly, <tt>\p{}</tt> matches a character's Unicode <i>script</i>. The +following scripts are supported: <i>Arabic</i>, <i>Armenian</i>, +<i>Balinese</i>, <i>Bengali</i>, <i>Bopomofo</i>, <i>Braille</i>, +<i>Buginese</i>, <i>Buhid</i>, <i>Canadian_Aboriginal</i>, <i>Carian</i>, +<i>Cham</i>, <i>Cherokee</i>, <i>Common</i>, <i>Coptic</i>, +<i>Cuneiform</i>, <i>Cypriot</i>, <i>Cyrillic</i>, <i>Deseret</i>, +<i>Devanagari</i>, <i>Ethiopic</i>, <i>Georgian</i>, <i>Glagolitic</i>, +<i>Gothic</i>, <i>Greek</i>, <i>Gujarati</i>, <i>Gurmukhi</i>, <i>Han</i>, +<i>Hangul</i>, <i>Hanunoo</i>, <i>Hebrew</i>, <i>Hiragana</i>, +<i>Inherited</i>, <i>Kannada</i>, <i>Katakana</i>, <i>Kayah_Li</i>, +<i>Kharoshthi</i>, <i>Khmer</i>, <i>Lao</i>, <i>Latin</i>, <i>Lepcha</i>, +<i>Limbu</i>, <i>Linear_B</i>, <i>Lycian</i>, <i>Lydian</i>, +<i>Malayalam</i>, <i>Mongolian</i>, <i>Myanmar</i>, <i>New_Tai_Lue</i>, +<i>Nko</i>, <i>Ogham</i>, <i>Ol_Chiki</i>, <i>Old_Italic</i>, +<i>Old_Persian</i>, <i>Oriya</i>, <i>Osmanya</i>, <i>Phags_Pa</i>, +<i>Phoenician</i>, <i>Rejang</i>, <i>Runic</i>, <i>Saurashtra</i>, +<i>Shavian</i>, <i>Sinhala</i>, <i>Sundanese</i>, <i>Syloti_Nagri</i>, +<i>Syriac</i>, <i>Tagalog</i>, <i>Tagbanwa</i>, <i>Tai_Le</i>, +<i>Tamil</i>, <i>Telugu</i>, <i>Thaana</i>, <i>Thai</i>, <i>Tibetan</i>, +<i>Tifinagh</i>, <i>Ugaritic</i>, <i>Vai</i>, and <i>Yi</i>. + + # Unicode codepoint U+06E9 is named "ARABIC PLACE OF SAJDAH" and + # belongs to the Arabic script. + /\p{Arabic}/.match("\u06E9") #=> #<MatchData "\u06E9"> + +All character properties can be inverted by prefixing their name with a +caret (<tt>^</tt>). + + # Letter 'A' is not in the Unicode Ll (Letter; Lowercase) category, so + # this match succeeds + /\p{^Ll}/.match("A") #=> #<MatchData "A"> + +== Anchors + +Anchors are metacharacter that match the zero-width positions between +characters, <i>anchoring</i> the match to a specific position. + +* <tt>^</tt> - Matches beginning of line +* <tt>$</tt> - Matches end of line +* <tt>\A</tt> - Matches beginning of string. +* <tt>\Z</tt> - Matches end of string. If string ends with a newline, + it matches just before newline +* <tt>\z</tt> - Matches end of string +* <tt>\G</tt> - Matches point where last match finished +* <tt>\b</tt> - Matches word boundaries when outside brackets; + backspace (0x08) when inside brackets +* <tt>\B</tt> - Matches non-word boundaries +* <tt>(?=</tt><i>pat</i><tt>)</tt> - <i>Positive lookahead</i> assertion: + ensures that the following characters match <i>pat</i>, but doesn't + include those characters in the matched text +* <tt>(?!</tt><i>pat</i><tt>)</tt> - <i>Negative lookahead</i> assertion: + ensures that the following characters do not match <i>pat</i>, but + doesn't include those characters in the matched text +* <tt>(?<=</tt><i>pat</i><tt>)</tt> - <i>Positive lookbehind</i> + assertion: ensures that the preceding characters match <i>pat</i>, but + doesn't include those characters in the matched text +* <tt>(?<!</tt><i>pat</i><tt>)</tt> - <i>Negative lookbehind</i> + assertion: ensures that the preceding characters do not match + <i>pat</i>, but doesn't include those characters in the matched text + + # If a pattern isn't anchored it can begin at any point in the string + /real/.match("surrealist") #=> #<MatchData "real"> + # Anchoring the pattern to the beginning of the string forces the + # match to start there. 'real' doesn't occur at the beginning of the + # string, so now the match fails + /\Areal/.match("surrealist") #=> nil + # The match below fails because although 'Demand' contains 'and', the + pattern does not occur at a word boundary. + /\band/.match("Demand") + # Whereas in the following example 'and' has been anchored to a + # non-word boundary so instead of matching the first 'and' it matches + # from the fourth letter of 'demand' instead + /\Band.+/.match("Supply and demand curve") #=> #<MatchData "and curve"> + # The pattern below uses positive lookahead and positive lookbehind to + # match text appearing in <b></b> tags without including the tags in the + # match + /(?<=<b>)\w+(?=<\/b>)/.match("Fortune favours the <b>bold</b>") + #=> #<MatchData "bold"> + +== Options + +The end delimiter for a regexp can be followed by one or more single-letter +options which control how the pattern can match. + +* <tt>/pat/i</tt> - Ignore case +* <tt>/pat/m</tt> - Treat a newline as a character matched by <tt>.</tt> +* <tt>/pat/x</tt> - Ignore whitespace and comments in the pattern +* <tt>/pat/o</tt> - Perform <tt>#{}</tt> interpolation only once + +<tt>i</tt>, <tt>m</tt>, and <tt>x</tt> can also be applied on the +subexpression level with the +<tt>(?</tt><i>on</i><tt>-</tt><i>off</i><tt>)</tt> construct, which +enables options <i>on</i>, and disables options <i>off</i> for the +expression enclosed by the parentheses. + + /a(?i:b)c/.match('aBc') #=> #<MatchData "aBc"> + /a(?i:b)c/.match('abc') #=> #<MatchData "abc"> + +Options may also be used with <tt>Regexp.new</tt>: + + Regexp.new("abc", Regexp::IGNORECASE) #=> /abc/i + Regexp.new("abc", Regexp::MULTILINE) #=> /abc/m + Regexp.new("abc # Comment", Regexp::EXTENDED) #=> /abc # Comment/x + Regexp.new("abc", Regexp::IGNORECASE | Regexp::MULTILINE) #=> /abc/mi + +== Free-Spacing Mode and Comments + +As mentioned above, the <tt>x</tt> option enables <i>free-spacing</i> +mode. Literal white space inside the pattern is ignored, and the +octothorpe (<tt>#</tt>) character introduces a comment until the end of +the line. This allows the components of the pattern to be organised in a +potentially more readable fashion. + + # A contrived pattern to match a number with optional decimal places + float_pat = /\A + [[:digit:]]+ # 1 or more digits before the decimal point + (\. # Decimal point + [[:digit:]]+ # 1 or more digits after the decimal point + )? # The decimal point and following digits are optional + \Z/x + float_pat.match('3.14') #=> #<MatchData "3.14" 1:".14"> + +*Note*: To match whitespace in an <tt>x</tt> pattern use an escape such as +<tt>\s</tt> or <tt>\p{Space}</tt>. + +Comments can be included in a non-<tt>x</tt> pattern with the +<tt>(?#</tt><i>comment</i><tt>)</tt> construct, where <i>comment</i> is +arbitrary text ignored by the regexp engine. + +== Encoding + +Regular expressions are assumed to use the source encoding. This can be +overridden with one of the following modifiers. + +* <tt>/</tt><i>pat</i><tt>/u</tt> - UTF-8 +* <tt>/</tt><i>pat</i><tt>/e</tt> - EUC-JP +* <tt>/</tt><i>pat</i><tt>/s</tt> - Windows-31J +* <tt>/</tt><i>pat</i><tt>/n</tt> - ASCII-8BIT + +A regexp can be matched against a string when they either share an +encoding, or the regexp's encoding is _US-ASCII_ and the string's encoding +is ASCII-compatible. + +If a match between incompatible encodings is attempted an +<tt>Encoding::CompatibilityError</tt> exception is raised. + +The <tt>Regexp#fixed_encoding?</tt> predicate indicates whether the regexp +has a <i>fixed</i> encoding, that is one incompatible with ASCII. A +regexp's encoding can be explicitly fixed by supplying +<tt>Regexp::FIXEDENCODING</tt> as the second argument of +<tt>Regexp.new</tt>: + + r = Regexp.new("a".force_encoding("iso-8859-1"),Regexp::FIXEDENCODING) + r =~"a\u3042" + #=> Encoding::CompatibilityError: incompatible encoding regexp match + (ISO-8859-1 regexp with UTF-8 string) + +== Special global variables + +Pattern matching sets some global variables : +* <tt>$~</tt> is equivalent to Regexp.last_match; +* <tt>$&</tt> contains the complete matched text; +* <tt>$`</tt> contains string before match; +* <tt>$'</tt> contains string after match; +* <tt>$1</tt>, <tt>$2</tt> and so on contain text matching first, second, etc + capture group; +* <tt>$+</tt> contains last capture group. + +Example: + + m = /s(\w{2}).*(c)/.match('haystack') #=> #<MatchData "stac" 1:"ta" 2:"c"> + $~ #=> #<MatchData "stac" 1:"ta" 2:"c"> + Regexp.latch_match #=> #<MatchData "stac" 1:"ta" 2:"c"> + + $& #=> "stac" + # same as m[0] + $` #=> "hay" + # same as m.pre_match + $' #=> "k" + # same as m.post_match + $1 #=> "ta" + # same as m[1] + $2 #=> "c" + # same as m[2] + $3 #=> nil + # no third group in pattern + $+ #=> "c" + # same as m[-1] + +These global variables are thread-local and method-local variables. + +== Performance + +Certain pathological combinations of constructs can lead to abysmally bad +performance. + +Consider a string of 25 <i>a</i>s, a <i>d</i>, 4 <i>a</i>s, and a +<i>c</i>. + + s = 'a' * 25 + 'd' + 'a' * 4 + 'c' + #=> "aaaaaaaaaaaaaaaaaaaaaaaaadaaaac" + +The following patterns match instantly as you would expect: + + /(b|a)/ =~ s #=> 0 + /(b|a+)/ =~ s #=> 0 + /(b|a+)*\/ =~ s #=> 0 + +However, the following pattern takes appreciably longer: + + /(b|a+)*c/ =~ s #=> 26 + +This happens because an atom in the regexp is quantified by both an +immediate <tt>+</tt> and an enclosing <tt>*</tt> with nothing to +differentiate which is in control of any particular character. The +nondeterminism that results produces super-linear performance. (Consult +<i>Mastering Regular Expressions</i> (3rd ed.), pp 222, by +<i>Jeffery Friedl</i>, for an in-depth analysis). This particular case +can be fixed by use of atomic grouping, which prevents the unnecessary +backtracking: + + (start = Time.now) && /(b|a+)*c/ =~ s && (Time.now - start) + #=> 24.702736882 + (start = Time.now) && /(?>b|a+)*c/ =~ s && (Time.now - start) + #=> 0.000166571 + +A similar case is typified by the following example, which takes +approximately 60 seconds to execute for me: + + # Match a string of 29 <i>a</i>s against a pattern of 29 optional + # <i>a</i>s followed by 29 mandatory <i>a</i>s. + Regexp.new('a?' * 29 + 'a' * 29) =~ 'a' * 29 + +The 29 optional <i>a</i>s match the string, but this prevents the 29 +mandatory <i>a</i>s that follow from matching. Ruby must then backtrack +repeatedly so as to satisfy as many of the optional matches as it can +while still matching the mandatory 29. It is plain to us that none of the +optional matches can succeed, but this fact unfortunately eludes Ruby. + +The best way to improve performance is to significantly reduce the amount of +backtracking needed. For this case, instead of individually matching 29 +optional <i>a</i>s, a range of optional <i>a</i>s can be matched all at once +with <i>a{0,29}</i>: + + Regexp.new('a{0,29}' + 'a' * 29) =~ 'a' * 29 + |